In aesthetics, nimism is a particular kind of trope

Trope (literature)

A literary trope is the usage of figurative language in literature, or a figure of speech in which words are used in a sense different from their literal meaning...

 or symbol

Symbol

A symbol is something which represents an idea, a physical entity or a process but is distinct from it. The purpose of a symbol is to communicate meaning. For example, a red octagon may be a symbol for "STOP". On a map, a picture of a tent might represent a campsite. Numerals are symbols for...

 characterized by exaggeration.
The term is derived from Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 (nimis), "too much" and (nimietas), excess. Unlike a hyperbole

Hyperbole

Hyperbole is the use of exaggeration as a rhetorical device or figure of speech. It may be used to evoke strong feelings or to create a strong impression, but is not meant to be taken literally....

 or paradox

Paradox

Similar to Circular reasoning, A paradox is a seemingly true statement or group of statements that lead to a contradiction or a situation which seems to defy logic or intuition...

, it is not applied to linguistic or rhetorical phenomena only, but usually refers to other forms of disparity, e.g. disproportions in sculptures and paintings, or certain sorts of discrepancies in appearance or behaviour (in theatre plays, movies etc.).
Thus nimism, by means of symbolic parallels or analogies, is meant to help the reader, viewer etc. to see the truth. A negative nimism hints at something bad (a weakness, vice, sin, crime etc.), the less frequently used positive nimism indicates something good (a virtue or something heroic).

Examples

• King Duncan’s cloak, too wide for his murderous successor Macbeth. (Shakespeare)
• The feet of the two haughty stepsisters, too big (and bloody) to fit Cinderella’s tiny shoe, hence an obvious evidence of their wrong claim. (Brothers Grimm)
• Elizabeth Bennet’s look at the nude male sculptures in Darcy’s art gallery, cast much too long to conceal her sexual awakening. (Jane Austen)
• The proverbial “Qui s’excuse s’accuse” referring to an apology offered too early, i.e. untimely and not asked for, and thus revealing a bad conscience.